Perturbation theory without multipole expansion

[Ward Moortgat]

The original (unexpanded) Hamiltonian is quite complex. Perturbing this Hamiltonian is non-trivial, first the corrections might be large. Second, we could perturbe this Hamiltonian in a lot of different ways, thus we need to order these different kinds of perturbations in order of magnitude to correctly use perturbation theory.

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